Heat sealed pouches made from flexible sheet stock are now being used for packaging certain products which are stored under sterile conditions. Typical products packaged and used in this manner are certain foods and medical supplies. The packaging thus used is known generally as a retort pouch.
The first generation of retort pouches included certain adhesives, used for laminating the various layers. The adhesives had some residual monomer moieties after the adhesive was fully cured. These structures were rejected by governmental authorities because of the perceived possibility of migration of toxic monomers into packaged food product.
In more recent developments, retort pouch structures are made by various methods of adhering the layers to each other by one or more of several extrusion processes. Illustrative of these processes is one taught in U.S. Pat. No. 4,190,477. In the process described in that patent, biaxially oriented polyester is first adhesively mounted to metal foil on what is to be the outside of the package. This use of adhesive is considered acceptable because the foil serves an an effective barrier to migration of residual monomer, from the adhesive, to the inside of the package. A polypropylene-based heat sealing layer is separately extruded. A primer is applied to the side of the foil opposite the polyester. Finally the sealant layer and the primed foil are extrusion laminated together using an extrusion grade polypropylene-based polymer as the extrusion laminant.
Common to the requirements of retort pouch packaging is the requirement that the filled and sealed package be subjected to sterilizing conditions of relatively high temperature after the pouch is filled with product and sealed. Typical sterilizing conditions range in severity up to about 275.degree. F. with residence times at that temperature of as much as 30 minutes or more. Such conditions impose severe stresses on the packages. Many packaging structures provide excellent protection for the package contents at less severe conditions. For example, relatively simple packaging structures for packaging requiring the ability to withstand boiling water, such as at 212.degree. F. are readily available from several suppliers. When sterilizing conditions are required, however, most of these packages fail to survive the processing. Typically, problems are encountered with excessive weakening or failure of the heat seals about the periphery of the pouch. Also certain weaknesses or separations may develop between the layers in the multiple layer sheet structure.
In some sterile packaging applications, it is desirable to be able to visibly inspect the product. In these cases, a transparent packaging film is highly desirable, and foil based films are less desirable.
In other packaging applications, it is indeed important that metal foil be specifically excluded from the structure so that the completed package can be penetrated by microwave energy, such as for cooking food contained in the package.
Among these structures which have proven themselves capable of withstanding the sterilizing process, the sealant, or inner layer of the pouch is believed to be, in almost all cases, based on polypropylene, and in some cases, propylene copolymers. While pouches made with the propylene based polymers are functionally capable of surviving the sterilizing process, their sheet structure is relatively brittle and hard. Thus the pouches are somewhat susceptible to cracking and crazing if subjected to rough handling. There are also some limited problems with weakening of interlayer adhesion between the layers. Once the interlayer adhesion is weakened, the pouch is, of course, subject to further damage by limited abusive handling which an unweakened pouch could normally tolerate.
While pouches made with known sheet structures have achieved a limited degree of success, it is desirable to provide an improved sheet structure which is capable of surving intact the typical sterilizing processes. It is particularly desirable to have a sheet structure with an improved sealant layer structure. In some cases, it is desirable that the sheet structure be transparent. In still other cases it is specifically important that the sheet structure contain no significant metallic component.
Among the characteristics of improved sealant layer structure are that it should be less brittle than the propylene-based sealants. It should also survive the sterilizing process with good heat seal strength about the pouch periphery. Also the interlayer adhesion should remain strong enough that the several layers remain intact as a unit and mutually support each other under stress, particularly after the sterilization process.